Simultaneous visualization of instantaneous unburnt and preheating zones in turbulent premixed flames under transverse acoustic excitations

Abstract

Instantaneous unburnt and preheating zones of bluff-body stabilized turbulent premixed flames under transverse acoustic excitations were investigated using simultaneous planar laser-induced fluorescence (PLIF) of acetone and CH2O, as well as multi-point hot-wire measurements. The PLIF images show that the unburnt zone marked by acetone images, the preheating zone marked by CH2O images, and the pixel-by-pixel product of acetone/CH2O have an increasing distribution area when slowly enlarging the sound pressure level (SPL). Wrinkled and bent edges of the unburnt and preheating zone can be seen at conditions away from the flame blow-off in the presence of the transverse acoustic excitations, and their sizes and areas increase as the flame blow-off is approached. At conditions near the flame blow-off with enlarging SPL to 123 dB, the flame turns from side to side over time and a large scale of the acetone/CH2O regions can be observed to deflect inside the center product zone, implying that the cold reactants can enter the product zone from the unburnt/preheating zones. The unburnt/preheating mainstream presents strong wrinkles and partial fractures. Such a phenomenon indicates that the local extinction of the shear layer flame can also be facilitated due to the turbulent fluctuation enhanced by the transverse acoustic wave. For a low flow velocity, increasing variations of the unburnt and preheating zones in the presence of the transverse acoustic wave can be revealed. The curvature of the acetone PLIF shows that the unburnt zones are more likely to be wrinkled with an increasing SPL and flow velocity. The root-mean-squared velocity measurements stress that the transverse acoustic wave mainly affects the turbulent premixed flame by enhancing the turbulent fluctuations.